Lash adjuster

ABSTRACT

A lash adjuster is provided in which the friction coefficient between the pressure flanks is less likely to decrease over a long time use. 
     The lash adjuster  1  includes a nut member  10  having an internal thread  17  on its inner periphery, an adjusting screw  11  having an external thread  18  on its outer periphery which is in threaded engagement with the internal thread  17 , and a return spring  12  biasing the adjusting screw  11  in a direction to protrude from the nut member  10 , the external thread  18  and the internal thread  17  having pressure flanks  19  and  21 , respectively, for receiving an axial load that tends to push the adjusting screw  11  into the nut member  10 , wherein a satin finished surface is formed on the pressure flank  19  of the external thread  18 , and the satin finished surface has a higher surface hardness than the pressure flank  21  of the internal thread  17.

TECHNICAL FIELD

This invention relates to a lash adjuster mounted in a valve gear of an engine.

BACKGROUND ART

Known valve gears for moving a valve provided at an intake port or an exhaust port of an engine include one comprising a valve lifter vertically movably supported and adapted to be pushed down by a cam, thereby pushing down a valve stem (direct type valve gear), one comprising an arm pivotable about its central portion and adapted to be pushed up by a cam at one end thereof, thereby pushing down a valve stem at the other end (rocker arm type valve gear), and one comprising an arm pivotable about one end thereof and adapted to be pushed down at its central portion, thereby pushing down a valve stem at the other end (swing arm type valve gear).

In these valve gears, gaps between their component parts may change due to differences in thermal expansion between component parts, which may cause noise and compression leakage. Also, when the sliding parts of the valve gear become worn too, gaps between component parts of the valve gear change, which may also cause noise.

In order to prevent such noise and compression leakage, ordinary valve adjusters include a lash adjuster for absorbing gaps between component parts of the valve gear.

One known lash adjuster used in a direct type valve gear comprises a lifter body vertically slidably inserted in a guide hole formed in a cylinder head, a nut member fixed to the lifter body, an adjusting screw having an external thread on its outer periphery which is in threaded engagement with an internal thread formed on the inner periphery of the nut member, and a return spring biasing the adjusting screw in the direction to protrude downwardly from the nut member, the adjusting screw pressing the valve stem of the valve gear with its end protruding from the nut member (Patent document 1).

One known lash adjuster used in an arm type valve gear comprises a nut member inserted in a mounting hole formed in the bottom surface of the arm which pivots as the cam rotates, an adjusting screw having an external thread on its outer periphery which is in threaded engagement with an internal thread formed on the inner periphery of the nut member, and a return spring biasing the adjusting screw in the direction to protrude downwardly from the nut member, the adjusting screw pressing the valve stem of the valve gear with its end protruding from the nut member (Patent document 2).

One known lash adjuster used in a swing arm type valve gear comprises a nut member inserted in a mounting hole formed in a top surface of a cylinder head, an adjusting screw having an external thread on its outer periphery which is in threaded engagement with an internal thread formed on the inner periphery of the nut member, and a return spring biasing the adjusting screw in the direction to protrude upwardly from the nut member, the adjusting screw pivotally supporting the arm of the valve gear with its end protruding from the nut member (Patent document 3).

In these lash adjusters, as a cam rotates and a load that tends to push the adjusting screw into the nut member is applied to the lash adjuster, the pressure flank of the external thread of the adjusting screw is supported on the pressure flank of the internal thread of the nut member, so that the adjusting screw is axially fixed in position.

If the relative position between the arm and the valve stem changes due e.g. to thermal expansion of the valve gear, according to the degree of change in the relative position, the adjusting screw axially moves in the nut member while rotating, thereby absorbing gaps between component parts of the valve gear.

In these lash adjusters, when the pressure flank of the external thread of the adjusting screw and the pressure flank of the internal thread of the nut member become worn over a long time of use, the surfaces of the pressure flanks become smooth, so that the friction coefficient between the pressure flanks decreases. This may cause slip between the pressure flanks when the cam rotates and a load is applied to the lash adjuster that tends to push in the adjusting screw, thus allowing the adjusting screw to be pushed in, which in turn reduces the valve lift.

In order to suppress reduction in friction coefficient between the pressure flanks over a long time use, a lash adjuster is proposed in which a satin finished surface is formed on one of the pressure flank of the external thread and the pressure flank of the internal thread (Patent document 4).

With this arrangement, since the protrusions and the recesses of the satin finished surface are sufficiently high compared to the depth of wear, even when the pressure flank of the external thread and the pressure flank of the internal thread become worn over a long time use, the friction coefficient between the pressure flanks is less likely to decrease.

Patent document 1: JP Patent Publication 2003-227318A Patent document 2: JP Patent Publication 2006-132426A Patent document 3: JP Patent Publication 2005-273510A Patent document 4: JP Patent Publication 2005-127189A

DISCLOSURE OF THE INVENTION Object of the Invention

But even when a satin finished surface is formed on one of the pressure flanks, if its surface hardness is substantially equal to or lower than that of the other pressure flank, the satin finished surface tends to quickly become worn. Thus, it may be difficult to sufficiently suppress reduction in friction coefficient between the pressure flanks.

For these lash adjusters, it is required to reduce their axial length in order to increase the freedom of design of the engine. But if the axial length of the lash adjuster is reduced, the meshing length between the threads also decreases, which in turn increases the surface pressure that acts between the pressure flanks of the external thread and the internal thread. This quickens wear of the external thread and the internal thread.

The inventors of the invention wanted to develop a lash adjuster in which the wear between the eternal thread and the internal thread is small even if the meshing length between the external thread and the internal thread is short, and for this purpose, conducted experimental tests for many lash adjuster samples of which the backlashes between the respective external and internal threads are different in size from each other.

The object of the present invention is to provide a lash adjuster of which the friction coefficient between the pressure flanks of the external thread and the internal thread over a long time use, and to reduce wear between the external thread and the internal thread of the lash adjuster.

In order to achieve this object, the present invention provides a lash adjuster comprising a nut member having an internal thread on its inner periphery, an adjusting screw having an external thread on its outer periphery which is in threaded engagement with the internal thread, and a return spring biasing the adjusting screw in a direction to protrude from the nut member, the external thread and the internal thread having pressure flanks, respectively, for receiving an axial load that tends to push the adjusting screw into the nut member, wherein a satin finished surface is formed on one of the pressure flank of the external thread and the pressure flank of the internal thread, and wherein the satin finished pressure flank has a higher surface hardness than the other of the pressure flanks. As used herein, the satin finished surface is a surface comprising irregularly arranged protrusions and recesses.

The return spring may be a compression coil spring axially biasing the adjusting screw, or a torsion spring for applying torque to the adjusting screw that tends to protrude the adjusting screw from the nut member. If a compression coil spring is used as the return spring, the external thread and the internal thread may have a serration-shaped section, with the pressure flanks having a larger flank angle than clearance flanks of the external thread and the internal thread, respectively.

The satin finished surface may have a surface roughness Ra of 1.6 to 12.5. The satin finished surface can be formed by electric discharge machining or laser beam machining. But by using shot peening, it is possible to harden the pressure flank and thus increase its durability. The surface hardness of the satin finished pressure flank can be increased by e.g. coating it with a hard film so as to be higher than the surface hardness of the other pressure flank. The hard film may be a titanium nitride film, a chromium nitride film, a diamond-like carbon film or a ceramic film. Carbonitriding treatment or WPC treatment can also increase the surface hardness of the satin finished pressure flank so as to be higher than that of the other pressure flank.

The inventors of the present invention conducted experimental tests for many lash adjuster samples of which the backlashes between the respective external and internal threads are different in size from each other, and as a result, discovered that by setting the backlash between the external thread and the internal thread within the range of 0.1 to 0.4 mm, the external thread and the internal thread are extremely less likely to become worn, compared to the case where this backlash is larger than 0.4 mm.

The concept of the present invention can be applied to any of the following types of lash adjusters:

1) A lash adjuster for use in a direct type valve gear, the lash adjuster including a lifter body vertically slidably inserted in a guide hole formed in a cylinder head, wherein the nut member is fixed to the lifter body, and wherein the adjusting screw has an end protruding from the nut member and pressing the valve stem of the valve gear. 2) A lash adjuster for use in a rocker arm type valve gear, wherein the nut member is inserted in a mounting hole formed in a bottom surface of an arm that pivots as a cam rotates, and wherein the adjusting screw has an end protruding from the nut member and pressing the valve stem of the valve gear. 3) A lash adjuster for use in a swing arm type valve gear, wherein the nut member is inserted in a mounting hole formed in a top surface of a cylinder head, and wherein the adjusting screw has an end protruding from the nut member and pivotally supporting the arm of the valve gear.

ADVANTAGES OF THE INVENTION

With the lash adjuster according to the present invention, even if the pressure flank of the external thread becomes worn after a long-term use, since the protrusions and recesses of the satin finished surface are sufficiently high compared to the depth of wear, the surface of the pressure flank of the external thread is less likely to become smooth, so that the friction coefficient between the pressure flanks of the external thread and the internal thread is less likely to decrease. Further, since the satin finished pressure flank has a higher surface hardness than the other pressure flank, the satin finished surface is less likely to become worn. This makes it possible to effectively suppress reduction of the friction coefficient between the pressure flanks.

With this lash adjuster 1, since one of the pressure flank of the external thread and the pressure flank of the internal thread is satin finished, when the pressure flanks of the external thread and the internal thread move close to each other, oil film is less likely to form due to the squeeze effect. Thus, even while the temperature is low and the viscosity of lubricating oil is high, the internal thread of the nut member can quickly receive the axial load applied to the adjusting screw.

By using titanium nitride film, chromium nitride film, diamond-like carbon film or ceramic film as the hard film, even if oil is used to which organic molybdenum is added, a lubricating film containing molybdenum disulfide is less likely to form on the hard film. This further reliably prevents slip between the pressure flanks of the external thread and the internal thread.

By setting the backlash between the external thread the internal thread to be 0.4 mm or less, compared to the arrangement in which the backlash is larger than 0.4 mm, the external thread and the internal thread are extremely less likely to become worn. This makes it possible to shorten the meshing length between the external thread and the internal thread, which in turn makes it possible to reduce the axial length of the lash adjuster.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a valve gear including a lash adjuster of a first embodiment according to this invention.

FIG. 2 is an enlarged sectional view of the lash adjuster shown in FIG. 1.

FIG. 3 is an enlarged sectional view of the adjusting screw shown in FIG. 2, showing its surface region.

FIG. 4 is an enlarged sectional view of a modified example of FIG. 3 in which the internal thread of the nut member is coated with the hard film shown in FIG. 3.

FIG. 5 is an enlarged sectional view of the lash adjuster of FIG. 1, showing a backlash between its external and internal threads.

FIG. 6 is a front view of a valve gear including a lash adjuster of a second embodiment according to this invention.

FIG. 7 is an enlarged sectional view of the lash adjuster of FIG. 6, showing a backlash between its external and internal threads.

FIG. 8 is a front view of a valve gear including a lash adjuster of a third embodiment according to this invention.

FIG. 9 is a graph showing the relationship between the backlash values before and after an endurance test for a sample with a thread meshing length that is substantially equal to that of a conventional device.

FIG. 10 is a graph showing the relationship between the backlash values before and after an endurance test for a sample with a thread meshing length that is ½ that of a conventional device.

FIG. 11 is a graph showing the relationship between the backlash values before and after an endurance test for a sample with a thread meshing length that is ⅓ that of a conventional device.

FIG. 12 is a front view of a valve gear including a lash adjuster of a fourth embodiment according to this invention.

FIG. 13 is an enlarged sectional view of the lash adjuster shown in FIG. 12.

FIG. 14 is a front view of a valve gear including a lash adjuster of a fifth embodiment according to this invention.

FIG. 15 is an enlarged sectional view of the lash adjuster shown in FIG. 14.

FIG. 16 is a front view of a valve gear including a lash adjuster of a sixth embodiment according to this invention.

FIG. 17 is an enlarged sectional view of the lash adjuster shown in FIG. 16.

DESCRIPTION OF THE NUMERALS

-   1. Lash adjuster -   2. Cylinder head -   5. Valve stem -   9. Lifter body -   10. Nut member -   11. Adjusting screw -   12. Return spring -   13. Guide hole -   15. Cam -   17. Internal thread -   18. External thread -   19. Pressure flank -   20. Clearance flank -   21. Pressure flank -   22. Clearance flank -   27. Hard film -   31. Lash adjuster -   33. Arm -   35. Nut member -   36. Adjusting screw -   37. Return spring -   38. Mounting hole -   51. Lash adjuster -   52. Arm -   53. Nut member -   54. Adjusting screw -   55. Return spring -   56. Mounting hole -   76. Torsion coil spring -   b. Backlash

BEST MODE FOR EMBODYING THE INVENTION

FIG. 1 shows a valve gear including a lash adjuster according to a first embodiment of the present invention. This valve gear includes a valve 4 provided at an intake port 3 of a cylinder head 2, and a valve stem 5 connected to the valve 4. The valve stem 5 extends upwardly from the valve 4. An annular spring retainer 6 is fixed to the outer periphery of the valve stem 5 at its top end portion. A valve spring 7 biases the spring retainer 6 upwardly, thereby seating the valve 4 on a valve seat 8.

The lash adjuster 1 comprises a lifter body 9, a nut member 10, an adjusting screw 11, and a return spring 12. The lifter body 9 is vertically slidably inserted in a guide hole 13 formed in the cylinder head 2.

The lifter body 9 comprises a cylindrical portion 9 a and an end wall 9 b closing the top end of the cylindrical portion 9 a. Over the lifter body 9, a cam 15 is located which is carried on a camshaft 14. As the camshaft 14 rotates, a cam lobe 15 b that protrudes from the base circle 15 a of the cam 15 presses the top end of the end wall 9 b, thus pushing down the lifter body 9.

As shown in FIG. 2, the nut member 10 is fixed to the bottom surface of the end wall 9 b by means of a snap ring 16. The member 10 has an internal thread 17 on its inner periphery which is in threaded engagement with an external thread 18 formed on the outer periphery of the adjusting screw 11. The external thread 18 has a pressure flank 19 that receives pressure when a load is applied that tends to push the adjusting screw 11 into the nut member 10. The external thread 18 has a serration-shaped section with the pressure flank 19 having a larger flank angle than its clearance flank 20. The internal thread 17 has also a serration-shaped section with its pressure flank 21 for receiving pressure when a load is applied that tends to push the adjusting screw 11 into the nut member 10 having a larger flank angle than its clearance flank 22.

The end of the adjusting screw 11 protruding from the nut member 10 is in contact with a spacer 23. The spacer 23 is rotationally fixed to the nut member 10 by means of a retainer 24 fixed in position by the snap ring 16 so as to be vertically movable within a cutout 25 formed in the retainer 24.

An oil hole 26 vertically extends through the end wall 9 b of the lifter body 9 so that lubricating oil supplied onto the top surface of the end wall 9 b is introduced into the nut member 10 through this oil hole 26. Lubricating introduced into the nut member 10 lubricates the external thread 18 and the internal thread 17.

As shown in FIG. 3, the external thread 18 of the adjusting screw 11 is subjected to shot peening and then coated with a hard film 27. Thus, the pressure flank 19 forms a satin finished surface having a surface hardness higher than the surface hardness of the pressure flank 17. Preferably, the satin finished surface has a surface roughness Ra of 1.6 to 12.5 to ensure that the protrusions and recesses of the satin finished surface are higher than the depth of abrasion. The hard film 27 may be a titanium nitride (TiN) film, chromium nitride (CrN) film, diamond-like carbon (DLC) film, or ceramic film.

The pressure flank 21 of the internal thread 17 is smaller in surface hardness than the pressure flank 19 of the external thread 18. As shown in FIG. 5, between the external thread 18 and the internal thread 17, a backlash b (axial clearance) of 0.1 to 0.4 mm is provided.

As shown in FIG. 2, the return spring 12 is a compression spring mounted between the adjusting screw 11 and the end wall 9 b of the lifter body 9 in an axially compressed state. The return spring 12 has its top end supported by the end wall 9 b with its bottom end axially pressing the adjusting screw 11, thereby biasing the adjusting screw 11 in the direction to protrude downwardly from the nut member 10. As shown in FIG. 1, the end of the adjusting screw 11 protruding from the nut member 10 presses the top end of the valve stem 5 through the spacer 23.

Now the operation of the lash adjuster 1 is described.

When the engine is started and the camshaft 14 rotates, the end wall 9 b of the lifter body 9 is pushed down by the cam lobe 15 b of the cam 15. The valve 4 thus separates from the valve seat 8, opening the intake port 3. At this time, a load is applied to the adjusting screw 11 that tends to push it into the nut member. But since the pressure flank 19 of the external thread 18 is received by the pressure flank 21 of the internal thread 21, the adjusting screw 11 remains axially fixed in position.

As the camshaft 14 further rotates and the cam lobe 15 b moves past the end wall 9 b, the valve stem 5 rises under the biasing force of the valve spring 7 until the valve 4 is seated on the valve seat 8 and the intake port 3 is closed.

In a strict sense, when the cam lobe 15 b pushes down the arm, slight slip occurs between the pressure flanks 19 and 21 of the external thread 18 and the internal thread 17. But after the cam lobe 15 b has moved past the end wall 9 b and by the time the cam lobe 15 b moves back to the position of the end wall 9 b, the adjusting screw 11 moves back to its original position under the biasing force of the return spring 12, because the load on the adjusting screw 11 that tends to push it into the nut member is removed during this period.

When the distance between the cam 15 and the lifter body 9 increases while the engine is running, due to thermal expansion differences between component parts of the valve gear, including the cylinder head 2 and the valve stem 5, the adjusting screw 11 protrudes by a larger distance when the cam 15 further rotates after pushing down the lifter body 9 and the load that tends to push in the adjusting screw 11 has been removed than the distance by which the adjusting screw 11 is pushed in when the lifter body 9 is pushed down by the cam lobe 15 b of the cam 15. Thus in this state, every time the cam 15 rotates once, the adjusting screw 11 gradually protrudes, thus preventing formation of a gap between the base circle 15 a of the cam 15 and the end wall 9 b of the lifter body 9.

Conversely, when the contact surfaces of the valve 4 and the valve seat 8 become worn, even while the base circle 15 a of the cam 15 faces the end wall 9 b of the lifter body 9, the biasing force of the valve spring 7 acts on the adjusting screw 11, so that the adjusting screw 11 protrudes by a shorter distance when the cam 15 further rotates after pushing down the lifter body 9 and the load that tends to push in the adjusting screw 11 has been removed than the distance by which the adjusting screw 11 is pushed in when the lifter body 9 is pushed down by the cam lobe 15 b of the cam 15. Thus in this state, every time the cam 15 rotates once, the adjusting screw 11 is gradually pushed in. This prevents formation of a gap between the contact surfaces of the valve 4 and the valve seat 8.

With this lash adjuster 1, even if the pressure flank 19 of the external thread 18 becomes worn after a long-term use, since the protrusions and recesses of the satin finished surface are sufficiently high compared to the depth of wear, the surface of the pressure flank 19 of the external thread 18 is less likely to become smooth, so that the friction coefficient between the pressure flanks 19 and 21 of the external thread 18 and the internal thread 17 is less likely to decrease. This in turn prevents excessive slip between the pressure flanks 19 and 21 of the external thread 18 and the internal thread 17 when a load acts on the adjusting screw 11 that tends to push it into the nut member.

With this lash adjuster 1, since the pressure flank 19 of the external thread 18, which is a satin finished surface, has a higher surface hardness than the pressure flank 21 of the internal thread 17, the satin finished surface is less likely to become worn. This makes it possible to effectively suppress reduction of the friction coefficient between the pressure flanks 19 and 21.

With this lash adjuster 1, since the pressure flank 19 of the external thread 18 is satin finished, when the pressure flanks 19 and 21 of the external thread 18 and the internal thread 17 move close to each other, oil film is less likely to form due to the squeeze effect. Thus, even while the temperature is low and the viscosity of lubricating oil is high, the internal thread 17 of the nut member 10 can quickly receive the axial load applied to the adjusting screw 11.

With this lash adjuster 1, by using titanium nitride film, chromium nitride film, diamond-like carbon film or ceramic film as the hard film 27, even if oil is used to which molybdenum dithiocarbamate (MoDTC) or molybdenum dithiophosphate (MoDTP) is added (what is known as FM oil), a lubricating film containing molybdenum disulfide (MoS₂) is less likely to form on the hard film 27. This further reliably prevents slip between the pressure flank 19 of the external thread 18 and the pressure flank 21 of the internal thread 17.

With this lash adjuster 1, since the backlash b between the external thread 18 and the internal thread 17 is 0.4 mm or less, compared to the arrangement in which the backlash b is larger than 0.4 mm, the external thread 18 and the internal thread 17 are extremely less likely to become worn. This makes it possible to shorten the meshing length between the external thread 18 and the internal thread 17, which in turn makes it possible to reduce the axial length of the lash adjuster 1.

With this lash adjuster 1, since the backlash b between the external thread 18 and the internal thread 17 is 0.1 or over, compared to the arrangement in which the backlash b is less than 0.1 mm, the adjusting screw 11 can rotate smoothly, so that the lash adjuster can stably perform its expected function.

In this embodiment, the external thread 18 is coated with the hard film 27 to increase the surface hardness of the pressure flank 19 of the external thread 18 higher than the surface hardness of the pressure flank 21 of the internal thread 17. But instead, the external thread 18 may be subjected to carbonitriding treatment to increase the surface hardness of the pressure flank 19 of the external thread 18.

The satin-finished surface may be formed on the pressure flank 19 of the external thread 18 of the adjusting screw 11 by subjecting the external thread 18 to WPC treatment. WPC treatment is a shot peening treatment for heating the surface of the external thread 18 to a temperature equal to or higher than the A3 transformation point, e.g. a treatment in which shots of 40 to 200 μm are thrown at the surface of the external thread 18 at 100 m/minute. By this treatment, the residual austenite on the surface of the eternal thread 18 turns into martensite, and the surface structure of the external thread 18 is recrystallized and refined, so that it is possible to increase the surface hardness of the external thread 18 higher than that of the pressure flank 21 of the internal thread 17.

As shown in FIG. 3, the external thread 18 may be subjected to shot peening and then coated with the hard film 27, over the entire surface thereof. With this arrangement, a satin finished surface is also formed on the clearance flank 20 of the external thread 18, so that the satin finished clearance flank 20 also has a surface hardness higher than that of the clearance flank 22 of the internal thread 17. This makes it possible to maintain a sufficiently high friction coefficient between the clearance flanks 20 and 22 of the external thread 18 and the internal thread 17, even if the clearance flank 20 of the external thread 18 becomes worn over a long time of use.

This satin finished surface can be formed by electric discharge machining or laser beam machining. But by using shot peening as in the above embodiment, it is possible to harden the pressure flank 19 and thus increase its durability.

In the above embodiment, of the pressure flank 19 of the external thread 18 and the pressure flank 21 of the internal thread 17, a satin finished surface is formed on the pressure flank 19 of the external thread 18, and the satin finished pressure flank 19 has a higher surface hardness than the pressure flank 21 of internal thread 17. But instead, as shown in FIG. 4, the internal thread 17 may be subjected to shot peening to form a satin finished surface on the pressure flank 21 of the internal thread 17, and its surface hardness may be made higher than the pressure flank 19 of the external thread 18. In this case too, the internal thread 17 may be subjected to shot peening and then coated with the hard film 27, over the entire surface thereof. This makes it possible to maintain a sufficiently high friction coefficient between the clearance flanks 20 and 22 of the external thread 18 and the internal thread 17, even if the clearance flank 22 of the internal thread 17 becomes worn over a long time of use.

FIG. 6 shows a valve gear including the lash adjuster 31 according to the second embodiment of the present invention. Elements corresponding to those of the first embodiment are denoted by identical numerals and their description is omitted.

This valve gear includes an arm 33 having its central portion pivotally supported on a pivot shaft 32. The art 33 carries a roller 34 at one end thereof, and carries the lash adjuster 31 at the other end. The roller 34 is in contact with a cam 15 fixed to a camshaft 14 so that when the camshaft 14 rotates, the arm 33 pivots.

The lash adjuster 31 includes a nut member 35, an adjusting screw 36 and a return spring 37. The nut member 35 is mounted in a mounting hole 38 extending vertically through the arm 33, and has an internal thread 17 formed on the inner periphery thereof and in threaded engagement with an external thread 18 formed on the outer periphery of the adjusting screw 36.

As with the first embodiment, a satin finished surface is formed on the pressure flank 19 of the external thread 18 by shot peening, and the external thread 18 is coated with a hard film 27 so that the satin finished surface has a surface hardness higher than the pressure flank 21 of the internal thread 17. A backlash b is defined between the external thread 18 and the internal thread 17 (see FIG. 7), which measures within the range of 0.1 to 0.4 mm.

The nut member 35 has its top end protruding from the top surface of the arm 33, and a cylindrical cap 39 having a bottom is interference-fitted on and fixed to the protruding top end of the nut member 35. The cap 39 engages the top edge of the mounting hole 38, thereby preventing the nut member 35 from downwardly separating from the hole 38. The cap 39 is formed with an oil hole 40 extending vertically therethrough so that lubricating oil supplied onto the top surface of the arm 33 is introduced into the nut member 35 through the oil hole 40.

The nut member 35 has a flange 41 at its bottom end that abuts the bottom surface of the arm 33, thereby supporting any upward force applied to the nut member 35.

The return spring 37 is a compression coil spring mounted between the cap 39 and the adjusting screw 36 in an axially compressed state. The return spring 37 has its top end supported by the cap 39 and axially presses the adjusting screw 36 at its bottom end, thereby biasing the adjusting screw 36 in the direction to protrude downwardly from the from the nut member 35. The end of the adjusting screw 36 protruding from the nut member 35 presses the top end of the valve stem 5.

With this lash adjuster, as with the first embodiment, even if the pressure flank 19 of the external thread 18 becomes worn after a long-term use, since the protrusions and recesses of the satin finished surface are sufficiently high compared to the depth of wear, the surface of the pressure flank 19 of the external thread 18 is less likely to become smooth, so that the friction coefficient between the pressure flanks 19 and 21 of the external thread 18 and the internal thread 17 is less likely to decrease. Also, since the pressure flank 19 of the external thread 18, which is a satin finished surface, has a higher surface hardness than the pressure flank 21 of the internal thread 17, the satin finished surface is less likely to become worn. This makes it possible to effectively suppress reduction of the friction coefficient between the pressure flanks 19 and 21.

With this lash adjuster 31, since the pressure flank 19 of the external thread 18 is satin finished, when the pressure flanks 19 and 21 of the external thread 18 and the internal thread 17 move close to each other, oil film is less likely to form due to the squeeze effect.

With this lash adjuster 31, since the backlash b between the external thread 18 and the internal thread 17 is 0.4 mm or less, compared to the arrangement in which the backlash b is larger than 0.4 mm, the external thread 18 and the internal thread 17 are extremely less likely to become worn. This makes it possible to shorten the meshing length between the external thread 18 and the internal thread 17, which in turn makes it possible to reduce the axial length of the lash adjuster 31.

With this lash adjuster 31, since the backlash b between the external thread 18 and the internal thread 17 is 0.1 or over, compared to the arrangement in which the backlash b is less than 0.1 mm, the adjusting screw 11 can rotate smoothly, so that the lash adjuster can stably perform its expected function.

FIG. 8 shows a valve gear including the lash adjuster 51 according to the third embodiment of the present invention. Elements corresponding to those of the first embodiment are denoted by identical numerals and their description is omitted.

This valve gear includes an arm 52 having one end thereof pivotally supported by the lash adjuster 51 and the other end in contact with the top end of the valve stem 5. A cam 15 fixed to a camshaft 14 is provided over the arm 52 so that when the camshaft 14 rotates, the arm 52 pivots about the lash adjuster 51.

The lash adjuster 51 includes a nut member 53, an adjusting screw 54 and a return spring 55. The nut member 53 is received in a mounting hole 56 formed in the top surface of a cylinder head 2, and has an internal thread 17 formed on its inner periphery and in threaded engagement with an external thread 18 formed on the outer periphery of the adjusting screw 54.

As with the first embodiment, a satin finished surface is formed on the pressure flank 19 of the external thread 18 by shot peening, and the external thread 18 is coated with a hard film 27 so that the satin finished surface has a surface hardness higher than the pressure flank 21 of the internal thread 17. A backlash is defined between the external thread 18 and the internal thread 17, which measures within the range of 0.1 to 0.4 mm.

A bottom member 57 is fixed to the bottom end of the nut member 53. The return spring 55 is a compression coil spring mounted between the adjusting screw 54 and the bottom member 57 in an axially compressed state. The return spring 55 has its bottom end supported on the bottom member 57 and axially presses the adjusting screw 54 at its top end through a spring seat 58, thereby biasing the adjusting screw 54 in the direction to protrude upwardly from the from the nut member 53. The end of the adjusting screw 54 protruding from the nut member 53 is engaged in a recess 59 formed in the bottom surface of the arm 52, thereby pivotally supporting the arm 52.

With this lash adjuster, as with the first embodiment, even if the pressure flank 19 of the external thread 18 becomes worn after a long-term use, since the protrusions and recesses of the satin finished surface are sufficiently high compared to the depth of wear, the surface of the pressure flank 19 of the external thread 18 is less likely to become smooth, so that the friction coefficient between the pressure flanks 19 and 21 of the external thread 18 and the internal thread 17 is less likely to decrease. Also, since the pressure flank 19 of the external thread 18, which is a satin finished surface, has a higher surface hardness than the pressure flank 21 of the internal thread 17, the satin finished surface is less likely to become worn. This makes it possible to effectively suppress reduction of the friction coefficient between the pressure flanks 19 and 21.

With this lash adjuster 51, since the pressure flank 19 of the external thread 18 is satin finished, when the pressure flanks 19 and 21 of the external thread 18 and the internal thread 17 move close to each other, oil film is less likely to form due to the squeeze effect.

With this lash adjuster 51, since the backlash between the external thread 18 and the internal thread 17 is 0.4 mm or less, compared to the arrangement in which the backlash b is larger than 0.4 mm, the external thread 18 and the internal thread 17 are extremely less likely to become worn. This makes it possible to shorten the meshing length between the external thread 18 and the internal thread 17, which in turn makes it possible to reduce the axial length of the lash adjuster 31.

With this lash adjuster 51, since the backlash between the external thread 18 and the internal thread 17 is 0.1 or over, compared to the arrangement in which the backlash b is less than 0.1 mm, the adjusting screw 11 can rotate smoothly, so that the lash adjuster can stably perform its expected function.

In order to confirm that wear can be reduced by setting the backlash between the external thread 18 and the internal thread 17 of this lash adjuster 51 to 0.4 mm or less, many samples having different backlashes between the external thread 18 and internal thread 17 from each other were prepared for each of the following three types 1) to 3) of the lash adjuster 51. Each sample was mounted on an actual engine and subjected to an endurance test. The relationship between the backlashes before and after the endurance test was investigated.

1) Lash adjuster 51 of which the meshing length between the external thread 18 and the internal thread 17 is substantially equal to that of a conventional lash adjuster. 2) Lash adjuster 51 of which the meshing length between the external thread 18 and the internal thread 17 is ½ that of a conventional lash adjuster. 3) Lash adjuster 51 of which the meshing length between the external thread 18 and the internal thread 17 is ⅓ that of a conventional lash adjuster.

The test conditions are as follows:

Engine revolving speed: 6000 rpm

Engine displacement: 1500 cc

Engine oil: OW-20 (SAE viscosity)

Duration: 500 hours

As a result, it was confirmed that although for samples of which the meshing length between threads is substantially equal to that of a conventional lash adjuster, as shown in FIG. 9, the backlash scarcely increased after the endurance test irrespective of the size of the initial backlash, for samples of which the meshing length between threads is ½ or ⅓ that of a conventional lash adjuster, as shown in FIGS. 10 and 11, an increase in backlash was extremely small after the test for any sample of which the initial backlash was 0.4 mm or less, compared to samples of which the initial backlash was larger than 0.4 mm.

The test results thus indicate that even if the meshing length between the external thread 18 and the internal thread 17 is made shorter than in conventional arrangements in order to reduce the axial length of the lash adjuster 51, it is possible to extremely effectively reduce wear between the external thread 18 and the internal thread 17.

For samples of which the initial backlash was 0.4 mm or less, the maximum surface pressure that acts between the pressure flanks 19 and 21 of the external thread 18 and the internal thread 17 was measured. For samples of which the meshing length between threads is substantially equal to that of a conventional lash adjuster, the maximum surface pressure was 10 MPa, for samples of which the meshing length between threads is ½ that of a conventional lash adjuster, the maximum surface pressure was 20 MPa, and for samples of which the meshing length between threads is ⅓ that of a conventional lash adjuster, the maximum surface pressure was 30 MPa.

FIGS. 12 and 13 show a valve gear including the lash adjuster of the fourth embodiment according to the present invention. Elements corresponding to those of the first embodiment are denoted by identical numerals, and their description is omitted.

The adjusting screw 11 of this embodiment has an external thread 73 on its outer periphery that is in threaded engagement with an internal thread 72 formed on the inner periphery of nut member 10. The external thread 73 and the internal thread 72 have a vertically symmetrical trapezoidal section, and are configured to receive axial loads that tend to push the adjusting screw 11 into the nut member 10 on the pressure flanks 74 and 75 of the external thread 73 and the internal thread 72.

As with the first embodiment, a satin finished surface having a surface roughness Ra of 1.6 to 12.5 is formed on the pressure flank 74 of the external thread 73 by shot peening, and the external thread 73 is coated with a hard film so that the satin finished surface has a surface hardness higher than the pressure flank 75 of the internal thread 72. A backlash is defined between the external thread 73 and the internal thread 72, which measures within the range of 0.1 to 0.4 mm.

A torsion coil spring 76 is mounted between the adjusting screw 11 and the end wall 9 b of the lifter body 9. The torsion coil spring 76 has its top end engaged in an engaging groove 77 formed in the top surface of the nut member 10 and is thus rotationally fixed to the nut member. Its bottom end is engaged in an engaging hole 78 formed in the adjusting screw 11. Thus, when the coil spring 76 is twisted, it applies torque to the adjusting screw 11 that tends to protrude the adjusting screw 11 downwardly from the nut member 10. The end of the adjusting screw 11 protruding from the nut member 10 presses the top end of the valve stem 5 through a spacer 23.

With this lash adjuster 71, even if the pressure flank of the external thread 73 becomes worn, since the protrusions and recesses of the satin finished surface are sufficiently high compared to the depth of wear, the surface of the pressure flank of the external thread 73 is less likely to become smooth, so that the friction coefficient between the pressure flanks 74 and 75 of the external thread 73 and the internal thread 72 is less likely to decrease. Also, since the satin finished pressure flank 74 of the external thread 73 has a higher surface hardness than the pressure flank 75 of the internal thread 72, it is possible to slow down the progression of wear of the satin finished surface, which in turn makes it possible to effectively suppress reduction in friction coefficient between the pressure flanks 74 and 75.

With this lash adjuster 71, since the pressure flank of the external thread 73 is satin finished, when the pressure flanks 74 and 75 of the external thread 73 and the internal thread 72 move close to each other, oil film is less likely to form due to the squeeze effect.

In this embodiment, the torsion spring for applying torque to the adjusting screw 11 that tends to protrude the adjusting screw 11 from the nut member 10 is the torsion coil spring 76. But instead of the torsion coil spring 76, a spiral spring may be used.

FIGS. 14 and 15 show a valve gear including the lash adjuster 91 of the fifth embodiment according to this invention. Elements corresponding to those of the second embodiment are denoted by identical numerals and their description is omitted.

The nut member 35 is inserted in a mounting hole 92 formed in the bottom surface of the arm 33, and has an internal thread 93 on its inner periphery which is in threaded engagement with an external thread 94 formed on the outer periphery of the adjusting screw 36. The external thread 94 and the internal thread 93 have a vertically symmetrical triangular section, and are configured to receive an axial load that tends to push the adjusting screw 36 into the nut member 35 on their respective pressure flanks 95 and 96.

As with the second embodiment, a satin finished surface having a surface roughness Ra of 1.6 to 12.5 is formed on the pressure flank 95 of the external thread 94 by shot peening, and the external thread 94 is coated with a hard film so that the satin finished surface has a surface hardness higher than the pressure flank of the internal thread 93. A backlash is defined between the external thread 94 and the internal thread 93, which measures within the range of 0.1 to 0.4 mm.

A torsion coil spring 97 is mounted between the adjusting screw 36 and the inner bottom surface of the mounting hole 92. The torsion coil spring 97 has its top end engaged in a through hole 98 formed in the inner bottom surface of the mounting hole 92 and thus is rotationally fixed to the arm 33. Its bottom end is engaged in an engaging hole 99 formed in the adjusting screw 36. Thus, when the coil spring 97 is twisted, it applies torque to the adjusting screw 36 that tends to protrude the adjusting screw 36 downwardly from the nut member 35. The end of the adjusting screw 36 protruding from the nut member 35 presses the top end of the valve stem 5.

Engine oil splashed onto the top surface of the arm 33 is introduced into the mounting hole 92 through the hole 98, thereby lubricating between the external thread 94 and the internal thread 93.

With this lash adjuster 91, even if the pressure flank 95 of the external thread 94 becomes worn, since the protrusions and recesses of the satin finished surface are sufficiently high compared to the depth of wear, the surface of the pressure flank 95 of the external thread 94 is less likely to become smooth, so that the friction coefficient between the pressure flanks 95 and 96 of the external thread 94 and the internal thread 93 is less likely to decrease. Also, since the satin finished pressure flank 95 of the external thread 94 has a higher surface hardness than the pressure flank 96 of the internal thread 93, it is possible to slow down the progression of wear of the satin finished surface, which in turn makes it possible to effectively suppress reduction in friction coefficient between the pressure flanks 95 and 96.

With this lash adjuster 91, since the pressure flank 95 of the external thread 94 is satin finished, when the pressure flanks 95 and 96 of the external thread 94 and the internal thread 93 move close to each other, oil film is less likely to form due to the squeeze effect.

In this embodiment, the torsion spring for applying torque to the adjusting screw 36 that tends to protrude the adjusting screw 36 from the nut member 35 is the torsion coil spring 97. But instead of the torsion coil spring 97, a spiral spring may be used.

FIGS. 16 and 17 show a valve gear including the lash adjuster 111 of the sixth embodiment according to the present invention. Elements corresponding to those of the third embodiment are denoted by identical numerals, and their description is omitted.

The nut member 53 has an internal thread 17 on its inner periphery at its lower portion. The adjusting screw 54 comprises an externally threaded member 54A having an external thread 19 on its outer periphery that is in threaded engagement with the internal thread 17 on the nut member 53, and a pivot member 54B axially slidably fitted in the nut member 53. A disk spring 112 is mounted between the pivot member 54B and the externally threaded member 54A. The end of the pivot member 54B inserted in the nut member 53 is supported by the externally threaded member 54A through the disk spring 112.

When the engine cools down and there appear differences in shrinkage between component parts of the valve gear, the disk spring 112 is configured to be compressed, thereby absorbing such differences in shrinkage. This prevents a gap between the valve 4 and the valve seat 8 due to differences in shrinkage between component parts of the valve gear, thereby preventing compression leakage, when the engine is restarted.

As shown in FIG. 3, the external thread 18 is subjected to shot peening and then coated with a hard film. Thus, the pressure flank 19 of the external thread 18 forms a satin finished surface having a surface roughness Ra of 1.6 to 12.5. By coating the external thread 18 with the hard film, the satin finished surface has a surface hardness higher than the pressure flank 21 of the internal thread 17.

As shown in FIG. 17, a torsion coil spring 114 is mounted between the externally threaded member 54A and a bottom portion 113 of the nut member 53. The torsion coil spring 114 has its bottom end engaged in a through hole 115 formed in the bottom portion 113 of the nut member 53 and thus is rotationally fixed to the nut member. Its top end is engaged in an engaging hole 116 formed in the externally threaded member 54A. Thus, when the spring 114 is twisted, it applies torque to the externally threaded member 54A that tends to protrude the pivot member 54B from the nut member 53.

As shown in FIG. 16, the pivot member 54B has its end 117 that protrudes from the nut member 53 hemispherically shaped. The protruding end 117 is engaged in a recess 59 formed in the bottom surface of the arm 52 at its end, thereby pivotally supporting the arm 52.

An oil discharge hole 118 is formed in the inner bottom surface of the mounting hole 56 that communicates with the through hole 115. Thus, engine oil flowing from the top end surface of the nut member 53 into the nut member 53 through the gap between the external thread 18 and the internal thread 17 is discharged from the nut member 53 through the hole 115 and then through the oil discharge hole 118.

With this lash adjuster 111, even if the pressure flank of the external thread 18 becomes worn, since the protrusions and recesses of the satin finished surface are sufficiently high compared to the depth of wear, the surface of the pressure flank 19 of the external thread 19 is less likely to become smooth, so that the friction coefficient between the pressure flanks 19 and 21 of the external thread 18 and the internal thread 17 is less likely to decrease. Also, since the satin finished pressure flank 19 of the external thread 18 has a higher surface hardness than the pressure flank 21 of the internal thread 17, it is possible to slow down the progression of wear of the satin finished surface, which in turn makes it possible to effectively suppress reduction in friction coefficient between the pressure flanks 19 and 21.

With this lash adjuster, since the pressure flank 19 of the external thread 18 is satin finished, when the pressure flanks 19 and 21 of the external thread 18 and the internal thread 17 move close to each other, oil film is less likely to form due to the squeeze effect.

In this embodiment, of the pressure flank 19 of the external thread 18 and the pressure flank 21 of the internal thread 17, a satin finished surface is formed on the pressure flank 19 of the external thread 18. But instead, as shown in FIG. 4, the internal thread 17 may be subjected to shot peening to form a satin finished surface on the pressure flank 21 of the internal thread 17, and its surface hardness may be made higher than the pressure flank 19 of the external thread 18. In this case too, the internal thread 17 may be subjected to shot peening only on the pressure flank 21 of the internal thread 17 or over the entire surface thereof.

In this embodiment, the torsion spring for applying torque to the adjusting screw 54 that tends to protrude the adjusting screw 54 from the nut member 53 is a torsion coil spring. But instead of the torsion coil spring, a spiral spring may be used. 

1. A lash adjuster comprising a nut member (10) having an internal thread (17) on its inner periphery, an adjusting screw (11) having an external thread (18) on its outer periphery which is in threaded engagement with the internal thread (17), and a return spring (12) biasing the adjusting screw (11) in a direction to protrude from the nut member (10), said external thread (18) and said internal thread (17) having pressure flanks (19 and 21), respectively, for receiving an axial load that tends to push the adjusting screw (11) into the nut member (10), characterized in that a satin finished surface is formed on one of the pressure flank (19) of the external thread (18) and the pressure flank (21) of the internal thread (17), and that the satin finished pressure flank has a higher surface hardness than the other of said pressure flanks (19 and 21).
 2. The lash adjuster of claim 1 wherein said return spring (12) is a compression coil spring axially biasing the adjusting screw (11), said external thread (18) and said internal thread (17) having a serration-shaped section, with said pressure flanks (19 and 21) having a larger flank angle than clearance flanks (20 and 22) of the external thread and said internal thread, respectively.
 3. The lash adjuster of claim 1 wherein said return spring is a torsion spring (76) for applying torque to the adjusting screw (11) that tends to protrude the adjusting screw (11) from the nut member (10).
 4. The lash adjuster of claim 1 wherein said satin finished surface has a surface roughness Ra of 1.6 to 12.5.
 5. The lash adjuster of claim 1 wherein said satin finished surface is formed by shot peening.
 6. The lash adjuster of claim 1 wherein the satin finished pressure flank (19) is coated with a hard film (27) and has a higher surface hardness than the other pressure flank (21).
 7. The lash adjuster of claim 6 wherein said hard film (27) is one of a titanium nitride film, a chromium nitride film, a diamond-like carbon film and a ceramic film.
 8. The lash adjuster of claim 1 wherein the satin finished pressure flank (19) is subjected to carbonitriding treatment so that the satin finished surface flank (19) has a higher surface hardness than the other pressure flank (21).
 9. The lash adjuster of claim 1 wherein the satin finished pressure flank (19) is subjected to WPC treatment so that the satin finished surface flank (19) has a higher surface hardness than the other pressure flank (21).
 10. The lash adjuster of claim 1 wherein a backlash of 0.1 to 0.4 mm is defined between the external thread (18) and the internal thread (17).
 11. The lash adjuster of claim 1 further comprising a lifter body (9) vertically slidably inserted in a guide hole (13) formed in a cylinder head (2), wherein said nut member (10) is fixed to said lifter body (9), and wherein said adjusting screw (11) has an end protruding from the nut member and pressing a valve stem (5) of a valve gear.
 12. The lash adjuster of claim 1 wherein the nut member (35) is inserted in a mounting hole (38) formed in a bottom surface of an arm (33) that pivots as a cam (15) rotates, and wherein said adjusting screw (36) has an end protruding from the nut member (35) and pressing a valve stem (5) of a valve gear.
 13. The lash adjuster of claim 1 wherein the nut member (53) is inserted in a mounting hole (56) formed in a top surface of a cylinder head (2), and wherein said adjusting screw (54) has an end protruding from the nut member (53) and pivotally supporting an arm (52) of a valve gear. 